Chlorine is popular because it handles the three main jobs in keeping a swimming pool clean: It sanitizes (kills bacteria and germs), oxidizes (controls organic debris from perspiration and body oils), and deters algae. The chemical is unpopular because it has a strong odor, reddens eyes, and causes allergic reactions in some swimmers.

There are alternatives to chlorine including bromine, ionizers, and ozonators, though with each you’ll still need to use some chlorine. A fourth alternative is PHMB, which doesn’t require the use of any chlorine. All four have drawbacks, including cost.

Chlorine is relatively cheap. How much chlorine you’ll need depends on the size of your pool, length of the swim season, amount of use, and other factors. For a 20,000-gallon pool that’s open year-round, figure you’ll spend about $600 annually.

Bromine

What it is: Pool suppliers sometimes suggest bromine as a substitute for chlorine. It can be an acceptable alternative for those with allergic reactions to chlorine, although that’s not guaranteed since bromine is also in the same halogen chemical family. Aquatic specialist Alison Osinski believes 5% of the population has an allergy to chlorine.

How it works: Bromine does a fine job as a sanitizer, but it doesn’t oxidize as well as chlorine. Most homeowners rely on a hybrid version known as BCDMH tablets that are typically 66% bromine and 27% chlorine to tackle that job. Some people opt for a two-step process of combining bromine salt extracted from seawater with potassium peroxymonosulfate (a.k.a. oxygen shock) in the pool to create that same sanitizing/oxidizing power.

Pros/Cons: Bromine remains stable at high temperatures, which is why many technicians recommend it for spas more than swimming pools. It’s less irritating on mucus membranes than its chlorine cousin, although it still produces an odor. And if you use just bromine in the pool (not the BCDMH compound), it leaves the water a dull green color that foams up when you swim in it, because the oxidation process is weaker.

Cost: It’s more expensive to operate a pool with bromine. Figure you could spend up to twice as much as you would if you use chlorine only.

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Ionizers

What it is: Ionizers rely on two dissimilar metals—often copper (an algaecide) and silver (a sanitizer)—sent charged into the water as the sanitizer. The oxidizer is missing, so you’ll need a small amount of chlorine or bromine in the water to handle this cleaning aspect.

How it works: An ionizer is a device that uses a low-voltage DC current to send these two metals into the water. The positive charge attracts bacteria, germs, and algae, and the new, larger compounds they form are carried out in the filtration system.

Pros/Cons: Like bromine, an ionizer doesn’t irritate swimmers’ eyes and noses. It can substantially reduce the amount of chlorine required. Chlorine and an ionizer work together better than chlorine alone, says Osinski.

Yet, she still considers ionizers a poor choice. For starters, you only reduce the chlorine amount significantly if just a few people use the pool on a regular basis, there are few plants and landscaping in the area, and your air isn’t heavily polluted. High dirt levels are beyond what an ionizer can fight on the sanitation side.

Also, ionizers depend on moving water, so you must run the pool pump continuously to keep the sanitizing action in place. And the increased levels of metal in the water can stain the pool and turn swimmers’ hair and fingernail beds green.

Cost: About $300 for an ionizer that handles up to 40,000 gallons of water. Homeowners may need to replace the metals in the system as often as once a swimming season, at an average cost of $129. Also factor in the energy cost of running the pool pump around the clock.

Ozonators

What it is: An ozonator is a machine that attaches to the filtration plumbing line. It inserts ozone gas (an active form of oxygen) into the pool to react with impurities in the water.

How it works: There are two types of ozone generators: ultraviolet light and corona discharge. In a UV light system, special low-pressure vapor lamps installed on the water return line create ozone to kill pathogens as they float by. Corona discharge generators rely on an electrical arc to create ozone inside the generator. Again, this ozone kills pathogens in the filtration system.

Pros/Cons: Ozone generators can reduce chlorine usage up to 90%, and they use the same amount of electricity as a 60-watt light bulb when the filter pump is turned on, so the added energy demand is tiny.

Aquatic consultants say ozonators combined with chlorine are extremely effective as long as you circulate the water 24/7. One caveat: Ozonators run best on dry air, so if you live in a humid climate, expect performance to decline.

Cost: A typical ozone generator starts at $600 to handle 7,000 gallons of water; $1,200 to cover 25,000 gallons. Take into account the expense of the pool pump running continuously.

PHMB

What it is: There’s only one way to eliminate the use of chlorine completely: Switch your pool to the chemical compound PHMB, short for polyhexamethylene biguanide. Homeowners commonly know PHMB by the Baquacil and SoftSwim brand names.

How it works: PHMB disinfects by penetrating bacteria cell walls, causing them to burst from within. It then wraps those particles in a heavy gel, which sinks to the bottom of the pool, where the vacuum system sucks it up.

Pros/Cons: PHMB doesn’t oxidize, so you’ll need to use hydrogen peroxide for this. You’ll also need to use a separate algaecide and clean pool filters—yes, even the sand ones—every four to six weeks.

PHMB is kinder on swimmers’ skin and hair, easy on vinyl pool liners, and doesn’t require as much attention as other chemicals to keep in balance. However, because PHMB is incompatible with chlorine, you’ll need to first drain the pool.

Once you’re back up and running, make sure every bathing suit has been washed. Even traces of chlorine in suit fibers will react with PHMB. The result of the reaction: a yellowish vapor that’ll radiate from your bathing suit.

Cost: The cost for PHMB chemicals to maintain a 10,000-gallon pool for a 16-week summer season is about $725.